The present invention relates to a weighing instrument with body fat meter for measuring simultaneously a bioelectrical impedance between both feet of a person in standing position and a weight, and for calculating and estimating a body fat amount based on such physical features as a height, a sex, an age and the like which are inputted other than above measurements, and, in particular, relates to a weighing instrument with body fat meter which responds to variations in a contact impedance between an electrode for measuring impedance and a sole of foot as a measured body.
Body fat meter which utilizes the fact that the composition of a human body can be estimated by the use of an impedance between ends of human body which can be obtained by applying a feeble constant current to an end of a measured person""s body and measuring a voltage drop between electrodes (The American Journal of Clinical Nutrition, 41(4) 810-817 1985 xe2x80x9cAssessment of fat-free mass using bioelectrical impedance measurement of the human bodyxe2x80x9d) was proposed (by U.S. Pat. No. 4,008,721, JP 5-49050C, JP 7-51242A, etc.) and the products based on these proposals have been introduced into market. Among them, the product according to JP 5-49050C is introduced into market as an instrument which allows to estimate body fat with absolute ease, that is, a weighing instrument with body fat meter, wherein a flat metal electrode for measuring a bioelectrical impedance is attached to a position on a top surface of a loading board of a weighing instrument with which soles of both feet of a measured person come in contact when he gets on the weighing instrument, whereby, among factors for estimating the body fat amount, an impedance between ends of human body and a weight which vary in every measurement can be measured simultaneously and can be taken into calculation formulas.
In a conventional body fat meter, at first, such physical data as a height, a sex, a weight and the like are inputted through such an input device as a key switch and the like and are stored in a memory, and then an impedance measuring device is driven by a controller to output a bioelectrical impedance in an analogue form, and then said analogue impedance is converted into digital value by an A/D converter to be taken into an arithmetic processor, which calculates a body fat amount from the digital value of the bioelectrical impedance and the physical data such as the height and the like stored in the memory, and outputs to an indicator. Since a weight, different from other physical data, changes easily, and thereby it should be inputted every time for a measurement, a weighing instrument is installed so as for a weight of a measured person to be measured every time when an impedance is measured in the weighing instrument with the body fat meter.
This weighing instrument with the body fat meter has an alternation switch between an analogue output of the impedance measuring device and said A/D converter, and another end of the alternation switch is connected to an analogue output of the weighing instrument, and a control terminal of the alternation switch is connected to said controller, wherein, at first, an output of the weighing instrument is inputted into the A/D converter through the alternation switch to convert a weight value of a measured person into a digital value and to store said digital value in a memory, and then the alternation switch is switched to input an output of the impedance measuring device into the A/D converter to convert a measured value of the impedance into a digital value, so that the A/D converter is shared by the weighing instrument and the impedance measuring device (FIG. 1).
In addition, the bioelectrical impedance measuring device of said conventional weighing instrument with body fat meter employs four-terminal method in order to eliminate an influence of variation in a contact resistance between an electrode and a human body upon a measured value (FIGS. 2 and 3).
Electrodes A1, A2, B1 and B2 are arranged so that said electrodes come to contact with tiptoes and heels of both feet of the measured person when he gets on the loading plate of an electronic weighing instrument for measuring the weight of the measured person, and current terminals of a constant-current regulated AC power source with known current value of iR in 50 kHz are connected to the electrodes A1 and A2 and measurement terminals of AC voltmeter are connected to the electrodes B1 and B2. This system is designed so that little current would flow into the measurement terminals of the AC voltmeter.
A bioelectrical impedance is represented by ri, a contact impedance of the right tiptoe by rA1, a contact impedance of the left tiptoe by rA2, a contact impedance of the right heel by rB1, and a contact impedance of the left heel is represented by rB2.
A constant-current regulated alternating current iR flows from rA1 through ri and rA2 and returns to the current terminal without being leaked to rB1 and rB2.
At that time, the voltage drops made by rA1, ri and rA2 are shown as below respectively.
vA1=iRxc3x97rA1xe2x80x83xe2x80x83(1)
vi=iRxc3x97rixe2x80x83xe2x80x83(2)
vA2=iRxc3x97rA2xe2x80x83xe2x80x83(3)
Since little current flows into each measurement terminal of the AC voltmeter, voltage drops made by rB1 and rB2 could be counted to be zero, that is, the effects caused by rB1 and rB2 could be negligible, so that vi can be directly observed by the AC voltmeter.
From the equation (2), the internal impedance ri is calculated as:
ri=vi/iRxe2x80x83xe2x80x83(4)
so that said impedance ri can be derived from the observed value vi because iR is a known value.
The constant-current regulated AC power source comprises a constant-voltage regulated AC power source, a resistor R1 and an OP amplifier (FIG. 4).
An output of the constant-voltage regulated AC power source is connected to an end of the resistor R1 and another end of the resistor R1 is connected to a negative terminal of the OP amplifier. To the negative terminal is connected said electrode A1, to an output terminal of the OP amplifier is connected said electrode A2, and a positive terminal of the OP amplifier is connected to GND (0V). The negative terminal of the OP amplifier has the same potential as the positive terminal does as far as the output terminal is not saturated, and no current flow in from the negative terminal into the OP amplifier. Accordingly, the current flowing into the resistor R1 directly flows into the electrode A1 through the human body, reaches to the electrode A2 and is absorbed by the output terminal of the OP amplifier. When the output voltage of the constant-voltage regulated AC power source is v, the voltage between both ends of the resistor R1 is v, so that:
ri=v/R1xe2x80x83xe2x80x83(5)
that is, the known constant-current can be obtained because v and R1 are known values.
The AC voltmeter comprises a differential amplifier, a rectifier, a low pass filter and an A/D converter. At first, the voltage between the electrodes B1 and B2 is amplified into N-times thereof by the differential amplifier.
At that time, an output voltage of the differential amplifier v is represented as below.
v=Nxc3x97Vi=Nxc3x97iRxc3x97rixe2x80x83xe2x80x83(6)
When this output is inputted into the half-wave rectifier, the rectifier outputs only a positive portions of the AC voltage. This output is transformed into DC by the low pass filter and inputted into the A/D converter and then the digital values proportional to the internal impedance ri is obtained.
By these procedure, the bioelectrical impedance free from the contact impedance of the feet can be measured.
In order to perform a precise measurement as much as possible, however, this system is designed generally on the assumption that the contact impedance is small to some degree and the system is used with bare feet. The contact impedance of a sole is generally less than 1 kxcexa9, so that the maximum current value for measuring the impedance is assumed to be less than 1 mA in the design specifications.
When the peak voltage of said constant-voltage regulated AC power source is 0.8 V and the resistor R1 is 1 kxcexa9, the current value is derived to be 800 xcexcA from the equation (5).
When the bioelectrical impedance is 500 xcexa9, and each contact impedance is 1 kxcexa9, the voltage V0 of the electrode A2 is calculated as below.
V0=800 xcexcAxc3x97(rA1+ri+rA2)=2 V
Generally, the contact impedance of the sole is less than 1 kxcexa9, but, when the socks or stockings are put on the feet, it increases extremely.
If the contact impedance is assumed to be 10 kxcexa9, the voltage V0 of the electrode A2 would be calculated as below.
V0=800 xcexcAxc3x97(10kxcexa9+500xcexa9+10kxcexa9)=16.4 Vxe2x80x83xe2x80x83(8)
On the other hand, most body fat meter is designed to be portable and a circuit thereof is powered by a battery. Accordingly, a circuit voltage is limited to a degree of xc2x15 V. A voltage of the output terminal of the OP amplifier used in the constant-current regulated power source is within said range, so that, when the socks or stockings are put on the feet (that is, the contact impedance is 10 kxcexa9) as described above, the OP amplifier is saturated and the constant current cannot be applied thereto. As a result, ri is not a known value of constant current, and a wave form thereof is deformed and goes out of sinusoidal wave form. However, the AC voltmeter would detect said deformed and incorrect voltage output vi, and, using said vi value, the arithmetic processor would calculate the body fat amount to output it to the indicator.
As having been described above, the bioelectrical impedance measuring device of the conventional weighing instrument with body fat meter is designed on the assumption that the measured person gets on the measuring instrument by the bare feet, and, accordingly, when the socks or stockings are put on the feet and thereby the contact impedance between the electrode of the measuring device and the measured body becomes too large, the voltage drop between the electrode of the measuring device and the measured body increases due to the constitution of the system and it goes out of a normal operation range of the constant-current power source, so that a predetermined degree of current cannot be applied to the human body and thereby an accurate measurement cannot be performed.
An object of the present invention is to provide a weighing instrument with body fat meter which allows a body fat to be precisely and simply measured even if a contact impedance between a human body and an electrode for measuring a bioelectrical impedance increases as a result of wearing socks and the like.
A constant-voltage power source is employed as a current power source for measuring a bioelectrical impedance, and a current measuring device is installed between said constant-voltage power source and an electrode of a voltage supply terminal to measure a current flowing into a human body (hereafter referred to as internal influent current), and then an impedance between both feet is determined based on a measured value of the internal influent current and a potential difference between voltage measurement terminals (FIG. 8).
The internal influent current -power source operates within the normal operating range without being affected by the contact impedance value, and the internal influent current and the potential difference between voltage measurement terminals which are used as base data for calculating the impedance are measured accurately, so that the bioelectrical impedance can be precisely measured even if there exists large contact impedance.
According to an aspect of the present invention, there are provided two pairs of electrodes of a measuring device for measuring a bioelectrical impedance between both feet, said two pairs of electrodes being composed of a pair of electrodes A (A1, A2) which comes in contact with the tiptoes of both feet when a measured person gets on a loading board of an electronic weighing instrument for measuring a weight thereof and makes a feeble current flow into the human body thereof and a pair of electrodes B (B1, B2) which comes in contact with the heels of both feet thereof and measures a potential difference between both heels, and an input device for inputting such physical features as a height, an age, a sex and the like. An output of a sinusoidal wave oscillator is lead to the electrode A via a resistor R1 with known resistance value, and the internal influent current is derived from the potential difference caused by the voltage drop by the resistor R1. On the other hand, the potential difference due to the voltage drop between the electrodes B is taken out by a differential amplifier, and the output thereof is, after a wave forming and a rectifying processing being applied thereto and being converted into the direct current, processed by the A/D conversion and then is taken into an arithmetic processing section as a digital data of the bioelectric impedance by the use of a current value derived from the potential difference between both ends of the resistor R1. The arithmetic processing section calculates a body fat amount based on the inputted physical features such as the height, the age, the sex and the like, a measured or inputted weight value and the bioelectrical impedance, and then the output is indicated on an indicator installed on a top surface of the loading board.
There will now be described in detail preferred embodiments of the present invention with reference to the drawings.